The extrusion of plasticized material mixtures into cellular structures (i.e., honeycomb) requires a delicate balance of deformability (for shape molding) and structural integrity (for shape retention). Such mixtures can include inorganic ceramic powders, a binder component and a liquid component, the amounts of which are controlled to maintain low pressure, torque, and temperature during the extrusion process while creating a self-supporting body which is able to be handled upon formation.
The cellular structure of the extrudate can be formed such that cells near the periphery of the formed body can have a smaller or reduced cross-section compared to cells near the middle or center of the body. In an ideal or pristine cellular structure, pressure equilibrium is achieved between the internal pressure of each cell and the ambient or atmospheric pressure outside the body. In some extrusion processes, however, the extrudate is cut to a desired length by a cutting tool such as a saw blade. As the extrudate is cut, the blade can “smear” or collapse the more delicate outer cells of the structure. Those cells with a smaller cross-sectional area tend to be more susceptible to smearing or collapsing during the cutting step due to their smaller size.
If a cell near the periphery or outer layer of the extrudate collapses due to smearing, the pressure equilibrium between the collapsed cell and the ambient is lost. Since the extrudate has a length dimension, each cell forms an elongated channel defined by this length. Thus, as the pressure equilibrium is destroyed when the cell collapses, one end of the channel is sealed causing the rest of the channel to collapse inward. The collapsed cell, or channel, thereby forms a visual defect referred to as a “skin groove” along the length of the extrudate.
A skin groove is undesirable as it is a visual defect that many customers do not want in the extruded part. More importantly, the skin groove can be the source of a higher stress concentration, and in some instances, cracks or microfissures can form in the groove. As cracks or microfissures develop, the structural integrity of the formed part is reduced and may result in a failed part.
Conventional techniques that have been used to reduce or eliminate skin grooves, albeit with moderate to little success, include improved cutting processes, ventilating the skin via mechanical or other scoring methods, stiffening the batch material by reducing the batch water percentage, and gelling and/or drying the skin of the extrudate with infrared or microwave energy. One such example of stiffening the wet extruded ceramic body through the use of microwave energy is described in U.S. Publication Ser. No. 2005/0093209 to Bergman et al., which is hereby incorporated by reference in its entirety. While the use of microwave energy has achieved positive results in some instances, microwave energy can result in non-uniform heating patterns particularly at or near the outer layers of the extrudate.
Thus, a need exists for improving the extrusion process, and in particular, the structural integrity of the wet ceramic extrudate prior to being cut. It is also desirable to provide a system that improves the structural integrity of the extrudate at or near the periphery thereof through a uniform means.